Capable of growing up to 3m in height this terrestrial plant is
No. 9 on the world’s 100 most destructive species list.
Only the female of this plant is present in the UK, and consequently
it rarely produces viable seeds but a piece of rhizome (root) as
small as 0.7g or a piece of living stem as small as 10cm can grow
a new plant.
The best method of control is complete removal of all living material,
both above and below ground. However this can be extremely expensive
as established infestations will often have extensive root systems
and any contaminated material removed from site must be taken to
an appropriately licensed land fill site.
Where possible, it would be preferable to bury any soil containing
knotweed on site at least 5m below ground and cover with an appropriate
impermeable root barrier, before back filling with uncontaminated
soil.
Alternatively a programme of treatment with a Glyphosate based herbicide,
applied in early September on at least 3 consecutive years followed
by careful monitoring, can produce favourable results.
Giant Hogweed Heracleum mantegazzianum
Taking four years to mature Giant Hogweed can reach heights in excess
of 5m. Each plant can produce 50,000 seeds which can remain viable
in the soil for up to 15 years. Its noxious sap, released from tiny
hairs on its leaves and stem, can cause painful blistering of the
skin when combined with sunlight.
Digging out the root crown is an effective control.
Himalyan Balsam Impatiens glandulifera
Reaching heights of up to 3m Himalayan balsam is the tallest annual
in the UK. Most commonly found on river banks or damp areas its ‘exploding’ seed
pods are capable of scattering seeds up to a distance of 7m.
It is easily controlled by pulling or cutting, but this must be
rigorously followed up each year until the seed bank is exhausted.
Winter Heliotrope Petasites fragrans
A native of North Africa this plant is commonly found on road verges
and disturbed ground. Flowering in Dec/Jan and growing to a height
of 30cms, only the male plant is present in the UK and therefore
can only spread by vegetative means.
An application of a Glyphosate based herbicide in Feb/Mar just after
flowering is thought to be the best method of control.
Three Cornered Leek Alium triquatrum
Often mistaken for one of our native wild flowers, it is becoming
more and more common on our road verges and in our woodlands, out-competing
native species. It is easily identified by its distinctive triangular
stem and white, ‘bluebell like’, flowers.
Best controlled by physical removal of underground bulbs or cutting before
flowering.
Monbretia Crocosmia x crocosmiiflora
Distinctive bright orange flowers mark this plant from May through
to September. It is notoriously difficult to get rid of for gardeners
and conservationists alike.
The best control is the physical removal of the underground corms.
Spanish Bluebell
Capable of hybridising with our native bluebell, this hybrid is
out-competing our native bluebell and other native woodland flowers.
Digging out the bulbs is the best method of control
Parrot’s Feather Myriophyllum aquaticum
Originally from South America, only female plants of this aquatic
are present in the UK and it is still sold in many garden centres.
A small stem fragment is all that is required for this aquatic to
form a new plant. Capable of covering a large pond within a year,
this and all other non-native Myriophyllum species cause
enormous problems in our ponds, canals, lakes and reservoirs.
Resistant to most herbicides and shading, careful physical removal
is the best form of control, remove it, compost it, dry it and burn
it, or bury it, then monitor it closely. Please do not throw it away,
or put it near ponds or streams.
A good alternative is the native mare’s tail, Hippurius
vulgaris
Floating Pennywort Hydrocotyle ranuncloides
An aquatic plant from America, it has caused problems wherever it
has been introduced; Chile, Australia, Holland and the UK. Forming
dense interwoven mats, it is capable of doubling its wet weight in
as little as three days and of spreading up to 15m in one season.
Physical removal is the best form of control, remove it, compost
it, dry it and burn it, or bury it, then monitor it closely. Please
do not throw it away, or put it near ponds or streams.
Good alternative plants are Frogbit, Water Mint, and Marsh Marigold
Austrialian Swamp Stonecrop Crassula helmsii
Introduced from Tasmania in 1911, this plant grows around damp margins
of ponds and in water to a depth of 3m. A tiny fragment, as small
as one node (5mm), is all that is required to produce a new plant,
and with no apparent dormant period is capable of year round growth.
Shading out other aquatic vegetation, creating a poor, oxygen depleted
habitat for invertebrates and fish, it is now threatening the remaining
habitats of the rare Starfruit and Hampshire Purslane.
The best control method is to pull out by hand. This will need to be repeated
for at least two years. Alternatively, cover with black plastic sheeting for
six months, or fill in the pond and start again in two years.
Water Fern Azolla filiculoides
Originally from North America, this plant forms dense mats, completely
covering the water surface up to a depth of 15cm. A distinctive feature
of this plant, which is green in summer, is its brick red winter
colour. Spreading very quickly, it carpets lakes, ponds, ditches
and other slow moving waters. It reproduces vegetatively by budding
and sexually by producing spores, which will begin to grow the following
spring.
The best way to eradicate it is to keep scooping it off the water
and adding it to the compost heap, monitoring closely. It is a nitrogen
fixing plant and makes very good compost.
